how "far" the Sun has to/can be from the Ascendant, in order to be considered, traditionally, diurnal in a chart?
Do these "measures" have to do with the number of degrees of combustion (8°30') and under the sunbeams (17° — I know the number of degrees can vary)?
I am studying a natal chart where the Asc is at 26°45' Aries and the Sun is at 08°08' Taurus and just cannot decide the sect, hayz and everything related to diurnality/nocturnality.

The Sun must be above the horizon for it to be diurnal. The positions you describe show the Sun about 12 degrees below the horizon. It's a nocturnal chart. There is no orb. Above is diurnal. Below is nocturnal.

Keep in mind that Sun conjunct ascendant is not the same as sunrise due to atmospheric refraction. The time of sunrise and sunset will determine whether a chart is diurnal or nocturnal, not whether it is above or below the ascendant - descendant axis. The Sun can be more than a degree below the ascendant and still be diurnal._________________Curtis Manwaring
Zoidiasoft Technologies, LLC

The Sun close to the ascendant is on the cusp, but for us to determine whether it is a day/night chart, a day chart is one where the Sun is in the southern hemisphere, above the horizon line, while night is vice versa.

The Asc at 26°45' Aries is, roughly, 11 degrees away from the Sn at 08°08' Taurus, which means that the Sun is below the horizon line, making it a night chart. The Sun is never in sect, nor the Asc and I am not familiar with the term Hayz so, Iris, I'm afraid I cannot answer this part - perhaps Thomas can?

Keep in mind that Sun conjunct ascendant is not the same as sunrise due to atmospheric refraction. The time of sunrise and sunset will determine whether a chart is diurnal or nocturnal, not whether it is above or below the ascendant - descendant axis. The Sun can be more than a degree below the ascendant and still be diurnal.

Is not the traditional - astrological - defnition of night and day that by the position of the Sun being below or above the true horizon, signified by the AC-DC Axis?

Keep in mind that Sun conjunct ascendant is not the same as sunrise due to atmospheric refraction. The time of sunrise and sunset will determine whether a chart is diurnal or nocturnal, not whether it is above or below the ascendant - descendant axis. The Sun can be more than a degree below the ascendant and still be diurnal.

Is not the traditional - astrological - defnition of night and day that by the position of the Sun being below or above the true horizon, signified by the AC-DC Axis?

Did the ancients understand atmospheric refraction? I would assume that they probably went with whether it was daylight or not which the start of daylight is not the same as Sun conjunct the ascendant, but the Sun's disk will be about a degree (+/-) above the eastern horizon when it is exactly on the ascendant. You might try experimenting here to understand the affect of atmospheric refraction:

I asked the question because I had always thought that the below/above the horizon distinction was clear enough, until I learnt there are certain differences.
Maybe I was also misled by my own understanding of the term "light", as there is visibile "light" (enough to see clearly) in the morning before the disk of the Sun actually appears on the (unobstructed) horizon.
Anyway, considering only atmospheric refraction, there are about five minutes (of the hour) difference between the officially given sunrise time and the Sun conjunct the Ascendant and (even assuming there is some imprecision in the calculation) that would not allow the Sun in the chart in question to be above the horizon.

"Light" can and often does mean just what it says: the stuff that comes from the Sun that illuminates everything it touches. But there is another concept: in Genesis, God says, "Let there be light." Yet the Sun wasn't created for a few more days. This kind of light, the kind caricatured with a light bulb going off above a cartoon character's head, can also be what is meant.

However "diurnal" is a different word than "light." It means "of the day," and "the day" is defined as the time the Sun is above the horizon. The hair splitting about when this actually occurs rarely affects a chart. It can, but I've never seen one where it was a problem. The example given in the question was about the Sun being 12 degrees, or roughly 48 minutes of clock time, before sunrise. That isn't close enough to be considered as anything but a nocturnal chart.

I understand what "diurnal" means and in fact my association line was light-daylight-day chart. And this made me ask if the distance inside which planets are burnt could be the distance inside which the Sun counts as starting "touching" the Ascendant. Too much of combinative imagination on my part
So I will keep on sticking to the horizon line
Anyway, in the case I mentioned, the Sun arrives to the Ascendant 27 minutes (of clock time) later.
I hope you do not mind my hairsplitting but I have at least 3 (natal) charts in the archives with the Sun very near to the Ascendant (not 12 degrees as above but 1-2) but still below it and trying to learn and apply traditional astrology I think it is actually important (considering not only the Sun but the other planets as well).

Tom, I've gotten tech support calls because of the "bug" in the program that said the chart was diurnal when the Sun was about half a degree below the ascendant. What people don't realize is that the Sun's disk will appear above the horizon due to atmospheric refraction. Schmidt suggested that it should be that if any part of the Sun's disk is visible to an observer (with an ideal flat landscape such as the ocean for example) then the chart is diurnal, else nocturnal. However at sunset, the diurnal sect is a bit of a lame duck and near sunrise, the same for the nocturnal sect.

I would have been a lot easier to just program ascendant < Sun < descendant, but I was being too careful._________________Curtis Manwaring
Zoidiasoft Technologies, LLC

I hope you do not mind my hairsplitting but I have at least 3 (natal) charts in the archives with the Sun very near to the Ascendant (not 12 degrees as above but 1-2) but still below it and trying to learn and apply traditional astrology I think it is actually important (considering not only the Sun but the other planets as well).

Chris Brennan has a lecture for purchase that might be helpful in cases like these:

On horizons, there is more than one definition of 'horizon'. Here are some of them:

Visible Horizon: The junction between the sky and the land (or sea)

Celestial horizon: The plane that passes through the centre of the earth normal to (at right angles to) a line from the centre of the eath to the observer's zenith.

Geoidal horizon: The plane that passes tangential to the earth at the observer's position.

Sensible horizon: The plane that passes through the observer's eye parallel to the Celestial/Geoidal horizon.

Normally, in astrological charts, we calculate the celestial horizon. But if we are to figure that the chart becomes diurnal once the Sun is seen to rise above the visible horizon, then are we to consider the moment the upper limb of the Sun is seen? The moment the centre of the Sun is seen to be above the visible horizon? The moment the whole Sun is above the visible horizon?

And let us not forget that the visible horizon depends on the observer's altitude. The higher you are, the earlier the sun appears to rise, so the altitude of the location of the chart would need to be known! But there again, if there is a mountain between you and the Sun....

—Tom: when seeking a solution to a problem, the statistical measure of the importance of the problem does not give the answer to the specific question. I asked the question referring to a certain chart but also having in mind the other ones about which I still have some big question marks in my mind. Percentage rates do not help me to solve the problem.
—EA: the possibility of rectifying the chart I mentioned was one of the reasons I asked this question, in fact. Thank you for the link.
—Geoffrey: I hope to fish something with this can of worms :-) Are there answers to the questions you have put (as these are questions I ask myself as well).
—Astraea: I had this kind of idea as well, but how can it be astrologically applied? These are not (or rarely) times when the astrologer is physically present at the birth of a child, and thus the question remains (and this was part of my original intention to ask): seeing a computer-drawn chart, how can one establish if there was some visible light already? (And that is why I thought that maybe the 8°30' of combustion or the 17° degree of under the sunbeams could be applicable.)

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